Minimal reproduction:

echo 'at_least_one_million = lambda _: _ >= 1_000_000' > foo.py
ruff foo.py --unsafe-fixes --fix
cat foo.py

File:

at_least_one_million = lambda _: _ >= 1_000_000

After fix:

def at_least_one_million(_):
    return _ >= 1000000

Expected behavior was that it would preserve my underscores (unless another linting rule disables underscores in the number), i.e. something like this:

def at_least_one_million(_):
    return _ >= 1_000_000